The interplay between true eustatic sea-level changes, tectonics, and climatic changes: what is the dominating factor in sequence formation of the Upper Oligocene–Miocene succession in the eastern North Sea Basin, Denmark?

Global and Planetary Change 41 (2004) 15 – 30 www.elsevier.com/locate/gloplacha

The interplay between true eustatic sea-level changes, tectonics, and climatic changes: what is the dominating factor in sequence formation of the Upper Oligocene–Miocene succession in the eastern North Sea Basin, Denmark? E.S. Rasmussen Geological Survey of Denmark and Greenland, Thoravej 8, DK-2400 Copenhagen NV, Denmark Received 22 November 2002; accepted 14 August 2003

Abstract The Upper Oligocene – Miocene succession of the eastern North Sea is subdivided into six sequences on the basis of an integrated study of outcrops, boreholes, and seismic data. All available data and methodologies/criteria for the definition of sequences have been used. Datings of the sequences are based on palynology, micropalaeontology, and macropalaeontolgy, which give them a high confidence. The six sequences defined in this study do not correlate to the eight marked changes in true eustatic sea-level variation as indicated from oxygen isotope curves nor to the nine glacial maxima that have been recognised within the Miocene. The development of the individual sequences and the stacking pattern of the sequences is dependent both on true eustatic sea-level changes, relative sea-level changes (tectonics), and sediment supply. For example, a true eustatic sea-level fall as well as uplift may result in major progradation of a siliciclastic wedge. In the Miocene succession studied here, a distinct progradation in the lower Aquitanian was the result of a true eustatic sea-level fall. This resulted in deposition of a clean and widespread sand-rich clastic wedge. Marked progradation associated with an uplift of the hinterland in the Langhian resulted in sand-rich deposits alternating with coals. The interbedded coal seams were the result of a rising ground water table within the basin due partly to stable or even rising sea level and partly to local subsidence around faults. Similarly, a major transgression may be a result of a true sea-level rise or accelerated subsidence of a basin. In the case of the eastern North Sea Basin, accelerated subsidence was responsible for the major transgression in the Serravallian and Tortonian times, which is a period with a general climatic cooling. The frequency of true eustatic sea-level changes and relative sea-level changes (tectonic pulses) in the Miocene is of the order of 2.9 – 2.0 to 4 m.y., respectively. Both these intervals are within the time range of the studied sequences, which span 2 – 4 m.y. Consequently, but not surprisingly, neither of the two processes responsible for changes in relative sea level can be solely attributed to glaciations nor to tectonics. The climatic changes during the Late Oligocene – Miocene, which changed from cool temperate to subtropical with several reversals, did not influence the sediment yield. D 2003 Elsevier B.V. All rights reserved. Keywords: North Sea; Denmark; Sea-level changes; Tectonics; Climate

E-mail address: [email protected] (E.S. Rasmussen). 0921-8181/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.gloplacha.2003.08.004

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1. Introduction

2. Geological framework

The architecture of a sedimentary succession is dependent on the changes in relative sea level and sediment supply. Sea-level changes are either attributed to true eustatic sea-level variation, basically due to climatic changes and growth of ice caps at the poles (e.g. Vail et al., 1991; Abreu and Anderson, 1998) or to the changing morphology of the basin due to tectonics (Cloetingh, 1988; Van Balen et al., 1998). The tectonic forces affecting the shape of a sedimentary basin are numerous, e.g. intraplate stress, rifting, underplating, delamination, and phase transformation (Kitahara et al., 1966; Ito and Kennedy, 1971; Green and Ringwood, 1972; Cloetingh, 1988; Nøttvedt et al., 1995; Nielsen et al., 2002). The frequencies of these tectonic mechanisms are debatable, but intraplate stresses and rift pulses are believed to be of relatively high frequency (Cloetingh, 1988; Van Balen et al., 1998; Nøttvedt et al., 1995). Recently, a number of studies have related high frequency unconformity bounded sequences to tectonic movements; e.g. the turbidities in the Terbernas Basin can be related to tectonic pulses of the Miocene Betic Event (Pickering et al., 2001) and Paleocene mass flow deposits in the North Sea are also associated to a tectonic event (Galloway et al., 1993). The Miocene succession of the eastern North Sea provides a good example to test the interrelationship between eustatic sea-level changes and those caused by intraplate stresses, because a large amount is known about climatic changes during the Miocene compared to other periods that can be tested by independent methodologies, e.g. global sea-level curves based on oxygen isotope measurements (Prentice and Matthews, 1988; Zachos et al., 2001), variation in atmospheric carbon dioxide (Pagani et al., 1999), and floristic changes in Central Europe (Mai, 1967; Utescher et al., 2000). Conclusions about the structural evolution are on the other hand more subjective. However, on the basis of a detailed seismic study of data from the North Sea combined with a sedimentological study of the Miocene succession onshore Denmark, many tectonic pulses within the North Sea area can be unravelled. The aim of the study is to investigate the interrelationship between true eustatic sea-level changes, tectonics, and climatic changes and its consequence for the infill of the eastern North Sea Basin (Fig. 1).

The formation of the North Sea Basin was a result of several tectonic events (Ziegler, 1991; Vejbæk and Andersen, 1987; Vejbæk, 1992; Underhill and Partington, 1993; Clausen et al., 1999; Møller and Rasmussen, in press). Volcanism and rifting occurred during Permian and Triassic times (Ziegler, 1982) and were responsible for the formation of the major structural elements in the basin (Fig. 2). Regional subsidence occurred during the Early Jurassic due to thermal sagging after the Permo-Triassic tectonic event. A major dome was formed in the central part of the North Sea region in the Middle Jurassic due to the mid-North Sea plume (Ziegler, 1982; Ziegler, 1991; Underhill and Partington, 1993). Extensive rifting occurred in the Late Jurassic to Early Cretaceous. This formed the final part of the structural elements that are known in the Central Graben area. From the Early Cretaceous onwards, subsidence due to thermal contraction followed the rifting. The North Sea Basin underwent inversion during the Late Cretaceous and Early Paleocene. This is considered to be a result of the collision between Africa and Eurasia, the Alpine Orogeny. The North Sea area was further affected by the Alpine Orogeny in the Late Eocene and fault-controlled subsidence and movements of salt occurred in the mid-Oligocene (Savian Phase). Erosion and uplift of the margins and accelerated deposition and subsidence of the central part of the North Sea Basin took place in the Late Neogene and Quaternary (Kooi et al., 1991; Vejbæk, 1992; Japsen and Bidstrup, 2000; Japsen et al., 2002a,b). The North Sea Basin was filled from the margins as a result of the elevation and erosion of the hinterland during the Cretaceous and Early Paleocene. Large amounts of sediments were supplied from the Shetland Platform and Norway in the Paleocene and Eocene (Galloway et al., 1993; Martinsen et al., 1999). The progradation of major siliciclastic wedges from the Fennoscandian Shield can be recognised in the eastern North Sea from the Oligocene (Michelsen et al., 1998; Clausen et al., 1999), and in the southern and central parts of the North Sea, a large sediment influx has been recognised from the Late Miocene (Cameron et al., 1993; Overeem et al., 2001). The overall architecture of the Upper Oligocene and Miocene succession in the study area is charac-

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Fig. 1. Study area. Boreholes and seismic key lines are indicated.

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Fig. 2. Structural elements in the eastern North Sea Basin. From Bertelsen (1978).

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terised by progradation of three sand-rich sedimentary wedges during the Early to Early Middle Miocene (Rasmussen, 2001). The sand was deposited in shallow marine and terrestrial environments. These deposits dominated in the eastern part of the area (Larsen and Dinesen, 1959; Rasmussen, 1961, 1996, 1998; Friis et al., 1998), while the western part is dominated by clayey shelf to deep-marine deposits. Sand-rich sediments and clay-rich fully marine sequences characterise the Middle and Upper Miocene succession in the whole study area. These deposits were laid down in outer shelf to littoral depositional environments representing an overall shallowing upward pattern (Konradi, 1996). Resumed

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major progradation in the eastern North Sea is first recognised in Pliocene (Gregersen et al., 1998).

3. Data A total of 11 outcrops and 25 borings onshore and offshore have been investigated (Rasmussen and Dybkjær, 1999; Dybkjær et al., 2001). Most of these have been described lithologically and dated by dinoflagellates (Dybkjær et al., 1999, 2001; Dybkjær and Rasmussen, 2000). Other biostratigraphic datings included in the study are: Sorgenfrei (1958), Larsen and Dinesen (1959), Rasmussen (1961), Piasecki (1980),

Fig. 3. Seismic lines used in the study.

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Konradi (1996), and Laursen and Kristoffersen (1999). Nine seismic surveys distributed both onshore and offshore Denmark were available for the study (Fig. 3).

4. Sequence stratigraphy of the Upper Oligocene – Miocene succession The Upper Oligocene –Miocene succession in the Danish area is subdivided into six sequences, A to F (Fig. 4) (Dybkjær and Rasmussen, 2000; Rasmussen, 2001). A brief description of each sequence will follow. In most of the onshore area, the base of the lowermost sequence (A) is formed by a major hiatus separating Eocene marly sediments from Chattian glaucony-rich clay. This boundary correlates to a seismic unconformity in the North Sea area (Fig. 4b and c). The lower part of sequence A consists of greenish, glacony-rich clay. There is an increase in silt and interbedded sand layers and lenses upwards and beds of iron ooids may occur in the uppermost part. The upper boundary is placed at the top of an oolitic sand layer forming the roof of the sequence (Van Houten and Purucker, 1984) and locally a thin gravel layer marks the sequence boundary. At the boundary, there is a change in depositional environment from marine to brackish water (Larsen and Dinesen, 1959; Rasmussen and Dybkjær, 1999) that is seen throughout the study area and was a result of a global sealevel fall in the Aquitanian (Dybkjær and Rasmussen, 2000; Rasmussen et al., 2002). The sequence boundary in the North Sea area correlates to a major unconformity (Fig. 4b and c) where slope failure occurred locally (Huuse and Clausen, 2001). Sequence B is composed of organic-rich silty clay. The content of sand increases upwards and locally major sand-rich deltaic deposits occur (Fig. 4a). A massive fluvial sand was deposited in the basinal setting during the late stage of the development of Sequence B and the displacement of the shoreline was in the order of 100 km (Rasmussen, 1998). The upper boundary of Sequence B is formed by a major erosional surface over which a hiatus of

ca. 3.5 m.y. separates Sequence B from Sequence C in the northeastern part (Dybkjær and Rasmussen, 2000). The sequence boundary can be seen as a regional unconformity on seismic sections (Fig. 4b and c). The lower part of Sequence C consists of organicrich silty clay with local sandy sediments deposited in transgressive barrier complexes (Fig. 4a). The sequence becomes sandy upwards and the overall evolution of the sequence is a progradation of a shoreline towards the southwest. The upper boundary is marked by a change from sandy deposits to more fine-grained sediments. The boundary is locally characterised by incision where it is very sharp and marine sediments are overlain by fluvial deposits (Fig. 4a). The boundary can be seen as a regional seismic unconformity (Fig. 4b and c). The lower part of Sequence D is composed of organic-rich sediments. The sequence becomes more sand-rich upwards and in places gravelly. Sequence D is different from B and C in having a very high content of heavy minerals (high gamma peaks in the log pattern in Fig. 4a). Up to three layers of brown coal occur in the upper part of the sequence (Koch, 1989). The upper boundary is characterised by a transition from fluvial deposits to marine organic-rich sediments. At the boundary, a transgressive gravel layer occurs. The sequence boundary corresponds to the so-called mid-Miocene unconformity on seismic panels (Jordt et al., 1995; Michelsen et al., 1998; Huuse and Clausen, 2001) and locally slope failure is associated with this boundary. Sequence E consists of marine silty clay with a very high content of organic matter in the lower part. The sequence becomes clayey upwards and rich in glaucony. The upper part of the sequence contains goethite deposited in shallow water (Dinesen, 1976). The sequence boundary towards Sequence F is defined where goethite-rich deposits are overlain by marine clay. This sequence boundary is only tentatively indicated on seismic panels (Fig. 4b and c) due to a lack of dating of the Upper Miocene succession within the North Sea. Sequence F is composed of marine, shell-rich clay. The sequence becomes silty and sandy up-

Fig. 4. Seismic sections and a correlation panel show the overall architecture of the uppermost Oligocene – Miocene sequences from west to east. The location of the seismic lines and correlation panel are indicated in Fig. 1 and on the inserted figure in the lower right.

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wards. The sand-rich sediments were deposited as storm sand layers (Rasmussen and Larsen, 1989). The boundary is correlated in the North Sea area to a regional unconformity close to the Tortonian – Messinian boundary (Jordt et al., 1995; Michelsen et al., 1998). In the North Sea area, this sequence is up to 400 m thick, which is considerably more than in the onshore area where normally only ca. 25 m have been penetrated.

5. Evidence for tectonics in the eastern North Sea Basin Four tectonic phases can be recognised during the deposition of the Late Oligocene– Miocene sediments studied here. Fault movements and flexuring can be recognised at the base of the Upper Oligocene succession on

seismic lines from the study area (Fig. 4). Fault controlled subsidence and a formation of a major unconformity occurred at this boundary in Central Europe (Ziegler, 1982, 1991; Hager et al., 1998) where this tectonic phase is known as the Savian Phase and it seems to have affected the North Sea region too. Large amounts of coarse-grained sediments from the Fennoscandian Shield were supplied to the North Sea basin during the Late Oligocene (Michelsen et al., 1998). This indicates indirectly an uplift of the hinterland, especially because it follows the Savian tectonic phase. A detailed study of the Miocene sediments reveals that at certain levels within the succession events such as uplift or earthquakes had occurred in the hinterland. Within the lower Aquitanian nearshore deposits in East Jylland, slumping and listric growth faults occurs at a well-defined level (Mikkelsen, 1983; Rasmussen and Dybkjær, 1999). These sedimentary structures

Fig. 5. Seismic section from the North Sea showing tectonic movements of a salt structure. Note the truncation of Sequence C. The timing of this tectonic movement corresponds to the abrupt increase in the content of heavy minerals. Note also the onlap of Middle Miocene deposits on the dome. This phase correlates to the onset of Late Miocene subsidence.

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(secondary structures) may have been triggered by an earthquake. Similar sedimentary structures (slumping of washover fans and listric faults in tidal deposits) are, however, also common without any connection with earthquakes. What is interesting here is that they occur only at a well-defined stratigraphic level and not at other levels where similar sediments are widely distributed within the succession. A distinct increase of reworked Paleocene and Eocene, and to some extent Jurassic spores, pollen, and dinoflagellates occurred in the mid-Burdigalian (Dybkjær et al., 1999, 2001). A high supply of heavy minerals dominates the remaining part of the Miocene lower shoreface and beach deposits (Fig. 4a). A thick pile of coal deposits was laid down in the mid-Langhian (Early Middle Miocene) in association with old fault trends in Central Jylland (Koch, 1989), indicating renewed fault activity at this time. The fluvial deposits laid down in Central Jylland were deposited by braided river systems that most likely reflect an increased topography in the area, e.g. growth of nearby salt structures (Miall, 1996). It is under any circumstances unlikely that braided river systems should dominate in an area that has been tectonically quiet for a long geological period; meandering and anastomosing river systems should be common. The two tectonic phases, mid-Burdigalian and mid-Langhian, can be correlated to two distinct phases in the growth of salt structures in the North Sea (Fig. 5). In Fig. 5, two phases in the growth of the salt structures can be recognised; one in the mid-Burdigalian and one in the mid-Langhian. The mid-Langhian phase also marked a change in direction of the sediment influx to the North Sea, from a dominantly northerly and north-easterly supply to a dominantly easterly influx of sediments (Clausen et al., 1999). The extreme thickness of the Serravallian– Tortonian succession (sequences E and F) in the North Sea (Fig. 4b and c) occurred during a period of falling global sea level (see below), indicating an increased subsidence of the North Sea Basin in Late Miocene times. This subsidence was not only concentrated to the central North Sea basin, but included also the onshore area (Japsen and Bidstrup, 2000; Japsen et al., 2002a,b). However, due to Late Cenozoic (possibly Pliocene or Quaternary) tilting of the North Sea Basin, strong erosion took place in the eastern part of the basin and removed most of the Upper Miocene deposits.

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Thus, four tectonic pulses occurred during the Late Oligocene– Miocene each with a time interval of the order of 2.5 – 5 m.y.

6. Evidence for climatic changes and true eustatic sea-level variations The climate changed from subtropical to cool temperate during the Late Oligocene – Miocene in Central Europe. The climate was humid and dominated by wet summers (Mai, 1967; Lotsch, 1968; Utescher et al., 2000). The climate was warm temperate in the Late Oligocene (Fig. 6). Sea-level curves, based on oxygen isotopes, indicate a general rise in sea level during the latest Oligocene. Cooling occurred at the boundary to the Miocene and a period with ice growth took place in the Antarctic (Miller et al., 1991). This resulted in lowering of the sea level and a cooler period in Central Europe. A minor rise in sea level in the Early Miocene followed the period indicated by the ice growth maximum (Mi-1) (Fig. 6). This was succeeded by a dramatical lowering in temperature in the Late Aquitanian and Early Burdigalian during which sealevel also fall. This period correlates to the glacial maximum of Mi-1a (Miller et al., 1991). A general sea-level rise took place during the Middle and Late Burdigalian as indicated by the oxygen isotope curve (Fig. 6). In Central Europe, the Burdigalian was relatively warm (subtropical – warm temperate); however, the overall trend of the temperature in the Burdigalian decreased slightly (Mai, 1967; Lotsch, 1968), which is different from the global trend indicated by the oxygen isotope curves (Prentice and Matthews, 1988; Zachos et al., 2001) (Fig. 6). A distinct sea-level rise occurred in the Early Middle Miocene (Langhian). This period, the late part of the Langhian, is also referred to as the mid-Miocene climatic optimum (Zachos et al., 2001). A subtropical climate correlates to this period on the climatic curve of Central Europe (Lotsch, 1968; Utescher et al., 2000). A general lowering in the temperature for Central Europe followed this mid-Miocene climatic optimum throughout the rest of the Miocene. This is also indicated by the overall fall in eustatic sea level and in the high frequency of glacial maxima (Mi-3 to Mi-7).

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Fig. 6. Comparison of glacio-eustatic sea-level changes, climatical changes, timing of maximum glaciations, and tectonic pulses with the timing of sequences identified within the study area. Note that the hiatus around the sequence boundaries is indicated in the figure by a grey area and arrows in the right column indicate major regressive events. The numbers indicated in the column of tectonics refer to tectonic events: (1) Savian tectonic event; (2) slumping of sediments; (3) marked rework; and (4) thick coal deposits associated with fault movements. Chronostraligraphy and geochronology after Berggren et al., (1985a,b).

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The changes in climate both in Europe and globally clearly indicate that variations in the sea level occurred during the Late Oligocene and Miocene. The period of sea-level changes based on the simplified curve of Zachos et al. (2001) is 2.9 m.y. (see Fig. 6).

7. Discussion Three variables are important in the formation of sequences: eustatic sea-level changes, tectonics, and sediment yield. The origin and influence of these parameters on the formation of the Upper Oligocene –Miocene succession in the eastern North Sea basin are discussed below. 7.1. Absolute sea-level changes during the Late Oligocene and Miocene The most important cause for eustatic sea-level changes is the fixation of water masses at the poles by growth of ice-caps. Several periods with increased fixation of water at the poles have occurred in the geological past, the so-called ‘‘icehouse Earth’’. Such an icehouse Earth has existed in the Cenozoic since the latest Eocene (Abreu and Anderson, 1998) during which Upper Oligocene –Miocene succession studied here was laid down. The method to study the changing volume of ice at the poles is the oxygen isotope record, which has been performed by a number of researchers (Buchardt, 1978; Prentice and Matthews, 1988, 1991; Miller et al., 1991; Zachos et al., 2001). Two of these studies, that of Prentice and Matthews (1988) and Zachos et al. (2001), are shown in Fig. 6. These curves are further compared with the local climatic changes based on floristic changes in Central Europe also showed in Fig. 6 (Mai, 1967; Lotsch, 1968; Utescher et al., 2000). The correlation of these results is good, although there are minor discrepancies in the timing and variations in the changes. Nevertheless, it can be concluded that the Late Oligocene– Miocene was a period with true glacio-eustatic sealevel changes. These glacio-eustatic sea-level changes are strongly superposed on any other processes that caused sea-level variations, e.g. swelling and shrinking of mid ocean ridges, eruption of basalt from mantle plumes, breakup of continental super plates, and sublithospheric mantle convection and advection

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(Pitman, 1978; Heller et al., 1996; Gurnis, 1993; Burgess et al., 1997). 7.2. Tectonic pulses A number of geodynamic processes can be involved during the subsidence evolution of a basin, e.g. thermal relaxation, stretching/rifting, delamination, underplating, phase transformation, sediment loading, unroofing, and intraplate stress (Kitahara et al., 1966; Ito and Kennedy, 1971; Green and Ringwood, 1972; Cloetingh, 1988; Nøttvedt et al., 1995; Nielsen et al., 2002). These processes act on a different time scales. The low frequency processes are those of thermal relaxation, delamination, underplating, and phase transformation. Differential subsidence associated with sediment loading and unroofing at the margins may be both long- and short-termed depending on the climatic conditions and changes. High frequency tectonics may be related to rifting and intraplate stress (Cloetingh, 1988; Nøttvedt et al., 1995; Van Balen et al., 1998). The subsidence evolution of the North Sea Basin during the Cenozoic has been interpreted to be caused by several factors. Basically, the subsidence of the basin is related to thermal relaxation following the Jurassic – Early Cretaceous rift event (Møller and Rasmussen, in press). However, additional processes as phase transformation and intraplate stress have been proposed to cause accelerated subsidence during the Cenozoic (Kooi et al., 1991; Vejbæk, 1992; Van Balen et al., 1998). This study of the Upper Oligocene and Miocene succession suggests that the tectonic effect was periodic and therefore intraplate stress is a likely process in the development of the North Sea basin. The collision between the African Plate and European Plate during the Cretaceous and Paleogene is well known and its effects can be traced in northwest Europe. The major inversion in the North Sea during the Late Cretaceous and Paleocene was a consequence of the collision (Ziegler, 1982, 1991; Vejbæk and Andersen, 1987). Tectonic movements in the latest Eocene are known to have affected the southern part of the North Sea, e.g. in Belgium (Ziegler, 1982) and probably also the northeastern part of the North Sea (Clausen et al., 1999). This effect of the Alpine tectonics is very evident from

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folding and tilting of areas and has caused formation of distinct unconformities in northwest Europe. The timing of the unconformities and pulses in the Alpine Orogeny is evident and not for debate for the Paleocene –Eocene period. During the Neogene, the effects of the Alpine orogeny is less evident, because no marked tilting or folding can be recognised. However, the collision between Eurasia and Africa continued during the Neogene and caused especially the Betic event (ca. 17 m.y.) on the Iberian Peninsula (Ribeiro et al., 1990). The timing of tectonic pulses during the Betic event can, however, still be traced in the studied succession. It is unlikely that the effects of the collision between the African Plate and the Eurasian Plate totally stopped in the Neogene. The change in the development of the succession correlates to phases in the Alpine orogeny and, consequently, it must be considered as a mechanism influencing the evolution of the North Sea Basin in the Neogene too. In addition to the collision between the African Plate and the Eurasia Plate, tectonics of the North Atlantic possibly also played a role. However, the dating of compressional phases in the North Atlantic is not well constrained (Boldreel and Andersen, 1993). Therefore a direct comparison cannot be done, but a Middle Miocene compressional event has been suggested, e.g. by Boldreel and Andersen (1993). The change in rift systems in Iceland at 15 m.y. correlates fairly to the Langhian tectonic phase found in this study.

and resulted in marked changes in the composition of the sediments, e.g. sudden increase in heavy minerals and reworked dinoflagellates, indicates that movements had occurred in the hinterland that increased sediment supply. The changes in base level due to sea-level changes, however, also influenced the hinterland by creating a new equilibrium profile. Enhanced erosion occurs during a sea-level fall due to lowering of the base level (Posamentier and Vail, 1988). The latter, however, is believed to be of minor importance compared to relief formed during structural uplift and cannot fully explain the marked change in mineral composition of the sediments and the sudden increase of reworked dinoflagellates that have been observed in the studied succession. The climate during the Late Oligocene –Miocene changes from cold temperate to subtropical with several reversals during the period. The precipitation was relatively high and concentrated to the summer period (Utescher et al., 2000). However, there has been no report on any distinct changes in humidity during the changing climatic conditions. Therefore, it is not likely that the fluctuation in the climate has had any major consequences for the sediment yield because high sediment supply has been found under both cool temperate climate, e.g. Early Aquitanian and under subtropical climate, e.g. Early Langhian.

8. Conclusion 7.3. Sediment yield Sediment yield from the hinterland into the basin is one of the controlling factors resulting in progradation of sedimentary wedges. The sediment yield is dependent on tectonic uplift, sea-level changes, climate, and type of source rock. Elevation due to tectonics creates a relief that under the right climatic conditions, results in an increase in sediment supply to the basin. This may lead to regression even during a sea-level rise if the influx is high. This is a well-known phenomenon from rift basins (Prosser, 1993; Rasmussen et al., 1998) and in collision zones (Summerfield and Hulton, 1994). The discussion above about tectonics, where certain tectonic pulses have been recognised

A comparison between climatical caused processes, eustatic sea-level changes, climate variation including precipitation and tectonic pulses is listed in (Fig. 6). The transgression in the uppermost part of the Oligocene seems to be a result of an eustatic sealevel rise and marine deposits were laid down in the onshore area for the first time in ca. 10 m.y. The sequence boundary at the Oligocene/Miocene boundary correlates to the glacial maximum: Mi-1 (Miller et al., 1991), which has resulted in a sea-level fall. This change is also recorded in the floristic changes in Central Europe and, consequently, there is a strong indication of a climatic control of this sequence boundary. The sediments, however, indicate

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some earthquake activity close to the boundary, but the conclusion is uncertain. The subsequent rise in eustatic sea level in the Early Aquitanian is recorded by the deposition of Sequence B. The development of Sequence B during the Aquitanian, where a displacement of the shoreline of 100 km has been recorded, was a result of a true eustatic sea-level fall in the Late Aquitanian and Early Burdigalian. This is clearly indicated by isotope data, floristic changes and timing of the glacial maximum Mi-1a. During this period, there is no evidence for tectonics (Fig. 6). The succeeding transgression in the Burdigalian resulted in deposition of Sequence C that developed between the two glacial maxima: Mi-1a and Mi-1b. The glacial maximum Mi-1b correlates actually to the sequence boundary, but the upper boundary cannot be correlated with a climatic cooling in Central Europe or any distinct sea-level fall. The abrupt change in mineral composition of the overlying sequence (Sequence D) by having a distinct higher concentration of heavy minerals and a lot of reworked Paleocene and Eocene deposits indicate tectonics in the hinterland. Salt movements also occurred in the North Sea Basin at this time. It is thus most likely that tectonics played a part in the formation of the sequence boundary of Sequence D. The development of Sequence D seems to be the result of high sediment influx due to the uplift in the hinterland, and perhaps a minor sea-level fall, indicated by the isotope data, glacial maximum Mi-2, and a minor cooling in Central Europe. However, the high content of coal beds within this sequence clearly indicates that the ground water table was rising during deposition. Consequently, uplift of the hinterland followed by high sediment supply is a likely explanation of the development of Sequence D. A tectonic pulse in the Langhian is additionally documented by thick coal beds along older faults in Central Jutland and on seismic data from the North Sea, where movements of salt structures occurred in the Late Langhian. The deposition of a relatively thick package of Late Serravallian and Tortonian marine deposits (Sequences E and F) above continental sediments strongly indicates a regionally increased subsidence rate of the eastern North Sea Basin. The initial transgression recorded by deposition of sequence E correlates well to the distinct rise in sea level in the Late Langhian and to the so-called

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mid-Miocene climatic optimum, but the succeeding cooling of the climate and general fall in eustatic sea level is not in line with the observed development of the Upper Miocene succession both onshore and offshore. Consequently, increased subsidence of the eastern North Sea basin must have occurred in the Late Miocene.

Acknowledgements The author is indebted to the Carlsberg Foundation for financial support of the project and to the Counties of Jylland for access to data. Morten S. Andersen and Jim Chalmers are acknowledged for their valuable comments on the manuscript. Eva Melskens is acknowledged for preparation of the figures. I would also like to thank Jens Christian Olsen and Henrik Rasmussen for providing and loading of seismic data. Thanks to the reviewers A.D. Miall, R.T. van Balen and the editor S.A.P.L. Cloetingh for constructive criticism and valuable comments on the manuscript.

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